(a) Field of the Invention
The present invention relates to a transmission type illumination equipment for microscopes which is so adapted as to permit favorable illumination from high to low magnification levels through a simple adjusting procedure.
(b) Description of the Prior Art
Conventionally, illumination equipments for microscopes which are so adapted as to be switchable from high to low magnification levels have been disclosed by U.S. Pat. No. 3,799,645, etc. Such a illumination equipment has such an optical system as shown in FIG. 1A in which the reference numeral 1 represents a light source, the reference numeral 2 designates a collector lens, the reference numeral 3 denotes a field stop, the reference numeral 4 represents an aperture stop arranged on an image 1' of said light source formed with said collector lens 2, the reference numeral 5 designates a condenser lens and the reference numeral 6 denote surface of an object to be observed. The optical components shown in FIG. 1A compose an illumination system for a high magnification level. The reference numerals 7 and 8 represents an auxiliary lens and another aperture stop which are arranged outside the optical path of the illumination system. In order to use this illumination system for microscopy at a low magnification level, the auxiliary lens 7 and aperture stop 8 which are placed outside the optical path when said illumination system is set for high magnification level are inserted into the optical path, and a portion 5a of the condenser lens 5 is removed from the optical path with the other portion 5b only left in the optical path. Though such an illumination system is usable from high to low magnification levels, adjustable range is inherently limited for it. Especially in the low magnification range, the illumination system is unusable at ultra-low magnification levels, e.g. 1.times., due to the fact that it cannot provide a field of illumination having an area wide enough to cover the object surface, etc. though it is usable at low magnification levels down to 6.times. or so. For making this conventional illumination equipment usable at ultra-low magnification levels, the composition of the illumination system shown in FIG. 1B must be further modified. In such a case, however, it becomes impossible to compose a Kohler illumination system and, therefore, the equipment cannot be used in an illuminating condition in which the field stop and aperture stop operate effectively. The reason for this is that a telecentric illumination is required at a low magnification level and a lens having a sufficiently long focal length, for example, five to ten times as long as that of a condenser lens for a high magnification level is necessary as the condenser lens, and it is difficult to satisfy the above-mentioned condition when a lens group of a condenser lens system arranged relatively near object surface is employed for illumination for microscopy at a low magnification level.